Please see page 30 of the attached. On Lotus 907 engines later than spec 3, they moved the water outlet from the back of the inlet manifold to the rear of the cylinder head, in order to address a pre-detonation issue they were having on cylinder 4.

I believe this outlet should be connected to the heater. The other heater hose connection is to a t-piece off the bottom hose just behind the fan.